Question: Simplify the following expression: $z = \dfrac{7}{6a} - \dfrac{1}{9a}$
Solution: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $6a$ and $9a$ $\lcm(6a, 9a) = 18a$ $ z = \dfrac{3}{3} \cdot \dfrac{7}{6a} - \dfrac{2}{2} \cdot \dfrac{1}{9a} $ $z = \dfrac{21}{18a} - \dfrac{2}{18a}$ $z = \dfrac{21 -2}{18a}$ $z = \dfrac{19}{18a}$